1. Field of the Invention
The present invention relates to a sensorless and brushless DC motor control apparatus.
2. Description of the Related Art
A brushless DC motor, for example, a spindle motor is incorporated in a magnetic disk apparatus so as to rotate a magnetic recording medium. The brushless DC motor comprises a Hall element which constitutes a position sensor (i.e., a Hall sensor), and the magnetic pole of the rotor having a magnet is detected by the Hall sensor. If the brushless DC motor is stationary (i.e., if the rotor is not rotated), the position of the magnetic pole of the rotor can be detected by the Hall sensor. When the brushless DC motor is driven, the motor coils to which a current is supplied are determined in accordance with the position of the magnetic pole of the rotor, so that the brushless DC motor can be driven normally.
In recent years, a sensorless and brushless DC motor not having a sensor has been employed so as to minimize the size of the magnetic disk apparatus. However, when the sensorless and brushless DC motor is stationary (i.e., when the rotor is not rotated), the position of the magnetic pole of the rotor cannot be detected. Therefore, a current is first supplied to desired motor coils, for the detection of the position of the magnetic pole of the rotor, and then normal driving control is performed.
If, as in the conventional technique, at the time of starting the sensorless and brushless DC motor, a current is supplied to desired motor coils, that is, desired phases are excited, the rotor will be rotated in the direction opposite to the desired direction, depending upon the position of the magnetic pole of the rotor. In this case, the rotor may continue to rotate in that opposite direction unless the excitation phases are switched at appropriate timings. In addition, the rotor may not rotate in one direction only, depending upon the position at which the magnetic pole of the rotor is located at the time of starting the sensorless and brushless DC motor. Thus, the rotation of the rotor cannot be controlled stably.
As mentioned above, the position of the magnetic pole of the rotor cannot be detected when the sensorless and brushless DC motor is stationary. Therefore, a rotor position signal is produced by the comparison between the back electromotive force of each motor coil and the voltage applied to the terminal used in common to all motor coils, and the switching timing of the excitation phases are determined in accordance with the rotor position signal. The excitation phases have to be switched, after the lapse of a predetermined time from a variation in the level of the rotor position signal. The suitable delay time is, for example, the time corresponding to a phase lag of 30.degree., i.e., the time corresponding to 1/12 of one revolution of the rotor. Therefore, the delay time has to be changed in accordance with the rotating speed of the rotor. In the conventional technique, this delay time is regulated by a delay circuit. With the delay circuit, however, it is difficult to change the delay time at the plurality of stages in accordance with the acceleration of the rotor. Since, therefore, the rotor cannot be accelerated with high efficiency, it requires a long time before the rotation of the rotor becomes constant.
Under these circumstances, there has been a demand for a sensorless and brushless DC motor control apparatus which enables a sensorless and brushless DC motor to be driven reliably and stably and which can control the acceleration of the rotor with high efficiency, thereby shortening the time required from the start of the rotor to the achievement of the constant rotation of the rotor.